Two-part mechanical interlock for electromagnetic contactors



p 1957 D. J. ARNEBERG 3,342,958

TWO-PART MECHANICAL INTERLOCK FOR ELECTROMAGNETIC CONTACTORS Filed on. 11, .1965 2 Sheets-Shet 1 flaw qfdmz beg Se t. 19, 1967 D. J. ARNEBERG 3 TWO-PART MECHANICAL INTERLOCK FOR ELECTROMAGNETIC CONTACTORS 2 Sheets-Sheet 2 Filed Oct. 11, 1965 United States Patent 3,342,958 TWO-PART MECHANICAL INTERLOCK FOR ELECTROMAGNETIC CONTACTORS Don J. Arneberg, Milwaukee, Wis., assignor to Square 1) Company, Park Ridge, IlL, a corporation of Michigan Filed Oct. 11, 1965, Ser. No. 494,615 16 Claims. (Cl. 200-50) This invention relates to electromagnetic contactors and to a mechanical interlock between two such contactors, the interlock insuring that the contacts of either one of the contactors open before the contacts of the other can be closed.

An object of the invention is to provide an improved mechanical interlock between two electromagnetic contactors.

Another object is to provide a machanical interlock for two electromagnetic contactors, the interlock having only two parts, one of which is pivotally mounted in the other, and the interlock being suitable for use between two contactors of the same or different physical sizes and electrical ratings.

In the drawings, FIG. 1 is a front elevational view, partially in section, of two electromagnetic contactors the casings and movable contact carriers of which are constructed to cooperate with a mechanical interlock constructed in accordance with the invention and mounted therebetween;

FIG. 2 is a top plan view of the contactors and mechanical interlock of FIG. 1;

FIG. 3 is a perspective view of the two parts of the mechanical interlock of FIGS. 1 and 2;

FIG. 4 is a fragmentary sectional view taken substantially along the line 4-4 of FIG. 5 and showing the relative positions of the parts when neither of the contactors'is energized, certain portions of one of the contactors being omitted for clarity;

' FIG. 5 is a fragmentary sectional view taken substantially along the line 55 of FIG. 2;

FIG. 6 is a view similar to FIG. 4, but showing the relative positions of the parts when the right-hand contactor of FIGS. 1 and 2 is energized first;

FIG. 7 is a view similar to FIG. 5, but showing the contact carrier assembly of the right-hand contactor in energized position;

FIG. 8 is a view similar to FIG. 4, but showing the relative positions of the parts when the left-hand contactor of FIGS. 1 and 2 is energized first; and

FIG. 9 is a fragmentary side elevational view of the left-hand contactor of FIGS. 1 and 2.

FIGS. 1 and 2 show a pair of contactors 10 and 12 constructed in accordance with the invention and more fully disclosed in application, Ser. No. 556,414, filed on June 9, 1966. The contactors 10 and 12 may be mounted in any suitable manner in spaced side-by-side correspondingly positioned relationship preferably so as to clamp therebetween a mechanical interlock 14 constructed in accordance with the invention. As shown, the contactors 10 and 12 are secured to a mounting plate 15 by a plurality of screws 16, the mounting plate being provided with a plurality of dimples 15a spaced respectively outwardly of the screws 16 so that as the screws 16 are tightened, the mounting plate 15 is resiliently deformed in a manner such that the top portions of the contactors 10 and 12 are urged toward each other to clamp the interlock 14 therebetween. The interlock 14 is best shown in FIG. 3 and includes a frame member 14:: and a locking member 14b pivotally mounted therein, both members prefer-ably being molded of suitable insulating material.

The contactor 10 includes a casing molded of electrically insulating material and having a base portion 10a and a cover portion 10b held together as by a pair of ICC screws 13. Similarly, the contactor 12 includes a casing having a base portion 12a and a cover portion 12b held together as by another pair of screws 13. The base portion 10a may be identical to the base portion 12a, but the cover portion 12b is larger than the cover portion 10b, since it is part of the contactor 12, which is made to accommodate larger load currents than the contactor 10 is made to handle. It will be understood that, if desired, a single size of the interlock 14 of my invention may be used between one of the larger contactors 12 and one of the smaller contactors 10, as shown, although more commonly it would be used between two of the larger contactors 12 or between two of the smaller contactors 10.

Each of the base portions 10a and 12a houses an encapsulated electrical coil 17 having a pair of terminals 18. Each of the base portions 10a and 12a also houses an E-shaped laminated magnetic core member 20 each of the outer legs of which is provided with a shading coil 21, and a magnetic armature member 22 adapted to be attracted into engagement with the respective core 20 upon energization of the respective coil 17. Each armature 22 is secured by a clip 24 to a pair of opposed depending legs of its respective movable contact carrier, a contact carrier 26 being mounted in the casing of the contactor 10 for reciprocal movement relatively thereto and a contact carrier 28 being mounted in the casing of the contactor 12 for reciprocal movement relatively thereto. As shown in FIG. 1, the lower portions of the contact carriers 26 and 28 may be identical, but the upper portion of the contact carrier 28 is larger than the upper portion of the contact carrier 26 because of the larger current rating of the contactor 12.

The contactors 10 and 12 are illustrated as having three poles, although it is to be understood that the principle of this invention is equally applicable to contactors of any number of poles. The cover portion 10b is provided with three pairs of terminals 30, one pair for each pole. Each terminal 30 is electrically connected to a stationary contact (not shown). The cover portion 12b is provided withthree pairs of terminals 32, one pair for each pole. Each terminal 32 is electrically connected to a stationary contact 33 (FIG. 7), the terminals 32 and contacts 33 being larger respectively than the terminals 30 and their associated stationary contacts, for greater current carrying capacity.

The contact carrier 26 of the contactor 10 carries three movable contact bridges 36, one of which is shown in FIGS. 5 and 7. Each bridge 36 is provided adjacent opposite ends respectively with a pair of movable con tacts (not shown), for engagement respectively with the pair of stationary contacts associated with the terminals 30 of the respective pole upon energization of the coil 17 of the contactor 10. The contact carrier 28 of the contactor 12 carries three movable contact bridges 38, one of which is shown in FIGS. 5 and 7. Each bridge 38 is provided adjacent opposite ends respectively with a pair of movable contacts 39, one of which. is shown in FIG. 7, for engagement respectively with the pair of stationary contacts 33 of the respective pole upon energization of the coil 17 of the contactor 12, the contacts 39 being larger than the contacts of the bridges 36.

The contact carrier 26 is provided on opposite sides respectively with a pair of camming and locking lugs 26a, one of which is shown in FIGS. 49. The cover portion 10b is provided on opposite sides respectively with a pair of aligned apertures 100, one of which is shown in FIGS. 5, 7, and 9. The lugs 26a on opposite sides of the contact carrier 26 are reciprocally movable respectively in the apertures 10:: and are offset in opposite directions, for a purpose hereinafter set forth, so as to be disposed substantially on opposite sides of a reference plane parallel to the direction of reciprocal movement of the contact carrier 26 and bisecting the apertures 100. Similarly, the contact carrier 28 is provided on opposite sides respectively with a pair of camming and locking lugs 28a, one of which is shown in FIGS. 4-8. The cover portion 12b is provided on opposite sides respectively with a pair of aligned apertures 120, one of which is shown in FIGS. 5 and 7. The lugs 28a 0n opposite sides of the contact carrier 28 are reciprocally movable respectively in the apertures 12c and are offset in opposite directions, for a purpose hereinafter set forth, so as to be disposed substantially on opposite sides of a'reference plane parallel to the direction of reciprocal movement of the contact carrier 28 and bisecting the apertures 12c, and in positions corresponding to the positions of the lugs 26a on the contact carrier 26'.

The frame member 14a (FIG. 3) is generally rectangular and is provided with eight projections 14c extending outwardly respectively from opposite sides thereof and respectively adjacent the four corners of each of the opposite sides thereof. When the interlock 14 is installed between the two contactors and 12, the projections 140 are received in suitable recesses provided respectively in the cover portions 10b and 12b to thereby support the frame member 14a, two of the lower projections 14a of FIG. 3 being received respectively in a pair of contoured portions 10d (FIG. 9) of a respective one of the apertures 10c, and the other two lower projections 140 being received respectively in a corresponding pair of contoured portions of a respective one of the apertures 120. Each of the opposite sides of the cover portion 1011 having the apertures 10c respectively therein is also provided with a pair of grooves 10a (FIGS. 2 and 9). The cover portion 12b is provided with similar pairs of grooves 12e (FIG. 2). Two of the upper projections 140 of FIG. 3 are received respectively in the respective pair of grooves 101;, and the other two upper projections 140 are received respectively in the respective pair of grooves 12e. An aperture comprising a circular portion 14d joined to a generally rectangular portion 14c extends completely through the frame member 14a, a pair of inwardly extending projections 14 being provided between the aperture portions 14d and 14c and defining a relatively narrow neck portion 14g connecting the aperture portions 14d and 14e.

The pivotable locking member 14b includes a first cylindrical portion 14h pivotal'ly receivable in the aperture portion 14d of the frame member 14a, the portion of the frame member 14a, defining the aperture portion 14d thus forming a bearing for the cylindrical portion 14h. The member 14b also includes What is shown as a second cylindrical portion 14k joined to the first cylindrical portion 14h by a connecting portion 14m, it being understood that the portion 14k need not necessarily be cylindrical. The length of the cylindrical portion 1411 and the corresponding dimension of the connecting portion 14m are preferably substantially equal to but less than the thickness of the frame member 14a exclusive of the projections 14c, while the length of the cylindrical portion 14k is preferably such that a pair of opposite end portions 1411 and 14p thereof project respectively beyond the projections 14c on opposite sides of the frame member 1411 when the member 1412 is centered in the frame member 14a axially of the cylindrical portions 14h and 14k. Preferably the opposite end faces of the cylindrical portion 14h are made slightly outwardly conical, as indicated by the shading in FIG. 3, to provide only point contact respectively with the contactors 10 and 12 and thereby insure that any frictional engagement respectively with the contactors 10 and 12 is kept at a minimum. The pair of oppositely extending projections Mn and 1417 are spaced from and extend parallel to the axis of pivotal movement of the locking member 14b. The diameter of the cylindrical portion 14k is considerably smaller than that of the cylindrical portion 14h, and the thickness of the connecting portion 14m perpendicular to a plane containing the axes of the two cylindrical portions 14h and 14k is small enough to permit a limited degree of pivotal movement of the member 14b, the projections 14; limiting movement in opposite directions by engagement respectively with opposite sides of the connecting portion 14m. The amount of pivotal movement of the member 14b is such that when it is in one extreme position, the cylindrical portion 14k is mainly within onehalf of the aperture portion 14s, and when it is in the other extreme position, the cylindrical portion 14k is mainly within the other half of the aperture portion 14e, as can be seen in FIGS. 6 and 8.

In FIGS. 4, 6, and 8, the cover portion 12b is omitted for clarity, the only portion of the contactor 12 shown being a section through the camming and locking lug 28a associated with the interlock 14. The aforementioned offset disposition of the lugs 26a and 28a causes the lug 26a and the lug 28a associated with the interlock 14 to be disposed substantially on opposite sides of a vertical (in FIGS. 4, 6 and 8) reference plane bisecting the frame member 14a and the apertures 10c and 12c. Each lug 26a of the contactor 10' is provided with a first relatively short camming surface 26b facing the reference plane and inclined relatively thereto at a relatively large acute angle and a second relatively long camming and locking surface 260 joining the surface 26b and facing the reference plane and inclined relatively thereto at a relatively small acute angle. Similarly, each lug 28a of the contactor 12 is provided with a first relatively short comming surface 28b facing the reference plane and inclined relatively thereto at a relatively large acute angle and a second relatively long camming and locking surface 28c joining the surface 28b and facing the reference plane and inclined relatively thereto at a relatively small acute angle.

In FIG. 4, the pivotable locking member 14b is shown in a central position and the lugs 26a and 28a are shown respectively in their lower or retracted positions corresponding to the deenergized condition of the respective coils 17.

FIG. 6 shows the lug 28a in raised or protracted position corresponding to the energized condition of the coil 17 of the contactor 12 and the closed-contacts position of the movable contact carrier 28. In the moving of the lug 28a to the position of FIG. 6, first the camming surface 28b engages the projection 14p at one end of the cylindrical portion 14k of the locking member 14b and pivots the member 14b counterclockwise in FIG. 6. After the projection 14p rides off the end of the camming surface 28b as the lug 28a continues to move toward fully protracted position, the camming and locking surface 28c pivots the locking member 1412 the rest of the way to its extreme counterclockwise position in FIG. 6. If an attempt is made to close the contacts of the contactor 10 by energizing its coil 17 while the coil 17 of the contactor 12 is energized and the lug 28a is in fully protracted position and the contacts of the contactor 12 are closed, the camming surface 26b engages the projection 14m at the other end of the cylindrical portion 14k and prevents the movable contact carrier 26 of the contactor 10 from moving to the closed-contacts position thereof. If the coil 17 of the contactor 12 is then deenergized while the coil 17 of the contactor 10 remains energized, the movable contact carrier 26 of the contactor 10 remains blocked by engagement of the lug 26a with the projection 1411 until the movable contact carrier 28 of the contactor 12 with its lug 28a is retracted sufficiently for the projection 14p to ride off the camming and locking surface 280, at which time the contacts of the contactor 12 will be open. The angularity of the surface 280 prevents binding of the movable contact carrier 28 in a protracted position due to pressure exerted by the projection 14p when the coil 17 of the contactor 10 is energized. Thus, it will be seen that the interlock 14 prevents closing of the contacts of the contactor until the contacts of the contactor 12 are open, but that the movable contact carrier 28 will not bind in a protracted position to completely prevent closing of the contacts of the contactor 10, as might occur if the surface 28c were vertical in FIGS. 4, 6, and 8.

FIG. 8 is a view similar to FIG. 6, but showing how the lug 28a is blocked when the movable contact carrier 26 is in closed-contact position and the lug 26a thereon is fully protracted. Thus, the interlock 14 prevents the contacts of either contactor from closing while those of the other are closed. It will be noted that the lugs 26a and 28a engage the cylindrical portion 14k on opposite sides of a plane containing the axis of pivotal movement of the locking member 14b and the axis of the cylindrical portion 14k.

I claim:

1. For use between a pair of spaced side-by-side correspondingly positioned movable contact carriers each mounted for reciprocal movement between first and second predetermined positions and having a camming and locking lug movable integrally therewith and disposed on the side thereof adjacent the other movable contact carrier, said lugs being offset from each other in a direction perpendicular to the paths of reciprocal movement of said movable contact carriers, a mechanical interlock for preventing movement of either one of said movable contact carriers to said first predetermined position thereof while the other of said movable contact carriers is in said first predetermined position thereof, said interlock comprising a frame member mountable between said movable contact carriers, and a locking member mountable in said frame member for limited pivotal movement with respect thereto, said locking member having a pair of axially aligned projections respectively disposed on opposite sides thereof, said projections being spaced from and extending axially in opposite directions parallel to the axis of pivotal movement of said locking member into interlocking relationship respectively with said lugs when said locking member is mounted in said frame member and said frame member is mounted between said movable contact carriers.

' -2. A mechanical interlock as claimed in claim 1, wherein said frame member is provided with an aperture extending completely therethrough from one side to an other side thereof, said aperture including a cylindrical first portion connected to a second portion, and said locking member includes a cylindrical portion and a connecting portion joining said cylindrical portion thereof to said projections on said locking member, the portion of said frame member defining the first cylindrical portion of said aperture forming a bearing for the cylindrical portion of said locking member, and the connecting portion of said locking member being disposed in said second portion of said aperture.

l 3. A mechanical interlock as claimed in claim 2, wherein opposite end faces of said cylindrical portion of said locking member are slightly outwardly conical.

4. A mechanical interlock as claimed in claim 1, wherein said frame member is provided with an aperture extending completely therethrough from one side to another side thereof, said aperture including a cylindrical first portion and a second portion connected by a neck portion, said frame member including a pair of inwardly extending projections defining said neck portion of said aperture, and said locking member includes a cylindrical portion and a connecting portion joining said cylindrical portion thereof to said, projections on said locking member, the portion of said frame member defining the cylindrical portion of said aperture forming a bearing for the cylindrical portion of said locking member, and the inwardly extending projections of said frame member forming a pair of stops limiting pivotal movement of said locking member in opposite directions by engagement respectively with said connecting portion of said locking member.

5. A mechanical interlock as claimed in claim 1, wherein said frame member is provided on opposite sides respectively with a plurality of outwardly extending projections for use in mounting said frame member between said movable contact carriers.

6. An assembly comprising a pair of spaced side-by-side correspondingly positioned movable contact carriers each mounted for reciprocal movement between first and second predetermined positions and having a camming and locking lug movable integrally therewith and disposed on the side thereof adjacent the other movable contact carrier, said lugs being offset from each other in a direction perpendicular to the paths of reciprocal movement of said movable contact carriers, and a mechanical interlock for preventing movement of either one of said movable contact carriers to said first predetermined position thereof while the other of said movable contact carriers is in said first predetermined position thereof, said interlock including a frame member mounted between said movable contact carriers and a locking member mounted in said frame member for limited pivotal movement with respect theretosaid locking mernber having a pair of axially aligned projections respectively disposed on opposite sides thereof, and said projections being spaced from and extending axially in opposite directions parallel to the axis of pivotal movement of said locking member into interlocking relationship respectively with said lugs.

-7. The assembly claimed in claim 6, wherein said frame member is provided with an aperture extending completely therethrough from one side to another side thereof, said aperture including a cylindrical first portion connected to a second portion, and said locking member includes a cylindrical portion and a connecting portion joining said cylindrical portion thereof to said projections on said locking member, the portion of said frame member defining the first cylindrical portion of said aperture forming a bearing for the cylindrical portion of said locking member, and the connecting portion of said locking member being disposed in said second portion of said aperture.

8. An assembly comprising a pair of spaced side-by-side correspondingly positioned movable contact carriers each mounted for reciprocal movement between first and second predetermined positions and having a camming and locking lug movable integrally therewith and disposed on the side thereof adjacent the other movable contact carrier, said lugs being disposed substantially on opposite sides of a first reference plane intersecting said movable contact carriers and extending parallel to the paths of reciprocal movement thereof, each lug having a first camming surface facing said first reference plane and inclined relatively thereto at a relatively large acute angle and a second camming and locking surface joining the first camming surface and facing said first reference plane and inclined relatively thereto at a relatively small acute angle, and a mechanical interlock for preventing movement of either one of said movable contact carriers to said first predetermined position thereof while the other of said movable contact carriers is in said first predetermined position thereof, said interlock including a frame member mounted between said movable contact carriers, and a locking member mounted in said frame member for limited pivotal movement with respect thereto about an axis in said first reference plane, said locking member having a pair of axially aligned projections respectively disposed on opposite sides thereof, said projections being spaced from and extending axially in opposite directions parallel to the axis of pivotal movement of said locking member into interlocking relationship respectively with said lugs for engagement respectively thereby on opposite sides of a second reference plane containing said axis of pivotal movement and the axis of alignment of said projections, whereby when either one of said movable contact carriers is in said first predetermined position thereof and an attempt is made to move the other of said movable contact carriers to said first predetermined position thereof, the second camming and locking surface of the lug on said one movable contact carrier engages the respective projection on the locking member on one side of said second reference plane to maintain the locking member substantially in one of its extreme positions, and the other projection on the locking member engages the first camming surface of the lug on said other movable contact carrier on the other side of said second reference plane to prevent movement of said other movable contact carrier to said first predetermined position thereof.

9. The assembly claimed in claim 8, wherein said movable contact carries respectively form parts of a pair of separate electromagnetic contactors.

10. The assembly claimed in claim 8, wherein said frame member is provided with an aperture extending completely therethrough from one side to another side thereof, said aperture including a cylindrical first portion and a second portion connected by a neck portion, said frame member including a pair of inwardly extending projections defining said neck portion of said aperture, and said locking member includes a cylindrical portion and a connecting portion joining said cylindrical portion thereof to said projections on said locking member, the portion of said frame member defining the cylindrical portion of said aperture forming a bearing for the cylindrical portion of said locking member, and the inwardly extending projections of said frame member forming a pair of stops limiting pivotal movement of said locking member in opposite directions by engagement respectively with said connecting portion of said locking member.

11. An assembly comprising a pair of spaced side-byside correspondingly positioned electromagnetic contactors each having a casing, an aperture in the side of the casing adjacent the other casing and aligned with the aperture in the other casing, a movable contact carrier mounted in the casing for reciprocal movement between open-contact and closed-contact positions, and a camming and locking lug on the movable contact carrier and disposed in the aperture for reciprocal movement therein, the lug being disposed on the opposite side of a first reference plane from the camming and locking lug on the other movable contact carrier, said first reference plane being parallel to the direction of reciprocal movement of the movable contact carriers and bisecting the aligned apertures, the lug having a first camming surface facing said first reference plane and inclined relatively thereto at a relatively large acute angle and a second camming and locking surface join-ing the first camming surface and facing said first reference plane and inclined relatively thereto at a relatively small acute angle, and a mechanical interlock for preventing movement of the movable contact carrier of either of said contactors to closed-contact position while the movable contact carrier of the other of said contactors is in closed-contact position, said interlock including a frame member mounted between said contactors, and a locking member mounted in said frame member for limited pivotal movement with respect thereto about an axis in said first reference plane, said locking member having a pair of axially aligned projections respectively disposed on opposite sides thereof, said projections being spaced from and extending axially in opposite directions parallel to the axis of pivotal movement of said locking member into interlocking relationship respectively with said lugs for engagement respectively thereby on opposite sides of a second reference plane containing said axis of pivot-a1 movement and the axis of alignment of said projections, whereby when the movable contact carrier of either one of said contactors is in closed-contact position and an attempt is made to move the movable contact carrier of the other of said contactors to closed-contact position, the second camming and locking surface of the lug on the movable contact carrier of said one contactor engages the respective projection on the locking member on one side of said second reference plane to maintain the locking member substantially in one of its extreme positions, and the other projection on the locking member engages the first camming surface of the lug on the movable contact carrier of said other contactor on the other side of said second reference plane to prevent movement of the movable contact carrier of said other contactor to closedcontact position.

12. The assembly claimed in claim 11, wherein said frame member is provided on opposite sides respectively with a plurality of outwardly extending projections and the casings of said contactors are provided respectively with a plurality of suitable recesses in which the projections on said frame member are respectively received.

13. The assembly claimed in claim 11, wherein said frame member is generally rectangular and each of a pair of opposite sides thereof is provided with four projections respectively adjacent the four corners, wherein each of the casings of said contactors is provided with a pair of grooves in which two of said projections on each of the opposite sides of said frame member are respectively received, and wherein the other two projections on each of the opposite sides of said frame member are respectively received in contoured portions of said apertures in said casings of said contactors.

14. The assembly claimed in claim 11, wherein said locking member includes a cylindrical portion the opposite end faces of which are slightly outwardly conical to insure only point contact of said opposite end faces respectively with the casings of said contactors.

15. The assembly claimed in claim 11, including a resiliently deformable mounting plate and fastening means securing the casings of said contactors to said mounting plate, the configuration of said casings and mounting plate and the disposition of said fastening means being such as to resiliently deform said mounting plate and effect clamping of said frame member by said casings of said contactors.

16. An assembly comprising a pair of spaced side-byside correspondingly positioned electromagnetic contactors each having a casing and a movable contact carrier reciprocably mounted therein, each of the casings of said contactors including an upper portion and a lower portion, mechanical interlock means disposed between and supported by the upper portion of the casings of said contactors and operatively connected to the movable contact carriers of said contactors, a, resiliently deformable mounting plate, and fastening means securing the lower portions of said casings to said mounting plate, the configuration of said lower portions and mounting plate and the disposition of said fastening means being such as to resiliently deform said mounting plate and effect urging of said upper portions toward said interlock means.

References Cited UNITED STATES PATENTS 3,240,889 3/ 1966 Lawrence et a1 200-50 ROBERT K. SCHAEFER, Primary Examiner.

J. R. SCOTT, Assistant Examiner. 

1. FOR USE BETWEEN A PAIR OF SPACED SIDE-BY-SIDE CORRESPONDINGLY POSITIONED MOVABLE CONTACT CARRIERS EACH MOUNTED FOR RECIPROCAL MOVEMENT BETWEEN FIRST AND SECOND PREDETERMINED POSITIONS AND HAVING A CAMMING AND LOCKING LUG MOVABLE INTEGRALLY THEREWITH AND DISPOSED ON THE SIDE THEREOF ADJACENT THE OTHER MOVABLE CONTACT CARRIER, SAID LUGS BEING OFFSET FROM EACH OTHER IN A DIRECTION PERPENDICULAR TO THE PATHS OF RECIPROCAL MOVEMENT OF SAID MOVABLE CONTACT CARRIERS, A MECHANICAL INTERLOCK FOR PREVENTING MOVEMENT OF EITHER ONE OF SAID MOVABLE CONTACT CARRIERS TO SAID FIRST PREDETERMINED POSITION THEREOF WHILE THE OTHER OF SAID MOVABLE CONTACT CARRIERS IS IN SAID FIRST PREDETERMINED POSITION THEREOF, SAID INTERLOCK COMPRISING A FRAME MEMBER MOUNTABLE BETWEEN SAID MOVABLE CONTACT CARRIERS, AND A LOCKING MEMBERS MOUNTABLE IN SAID FRAME MEMBER FOR LIMITED PIVOTAL MOVEMENT WITH RESPECT THERETO, SAID LOCKING MEMBER HAVING A PAIR OF AXIALLY ALIGNED PROJECTIONS RESPECTIVELY DISPOSED ON OPPOSITE SIDES THEREOF, SAID PROJECTIONS BEING SPACED FROM AND EXTENDING AXIALLY IN OPPOSITE DIRECTIONS PARALLEL TO THE AXIS OF PIVOTAL MOVEMENT OF SAID LOCKING MEMBER INTO INTERLOCKING RELATIONSHIP RESPECTIVELY WITH SAID LUGS WHEN SAID LOCKING MEMBER IS MOUNTED IN SAID FRAME MEMBER AND SAID FRAME MEMBER IS MOUNTED BETWEEN SAID MOVABLE CONTACT CARRIERS. 